Heavy oil is naturally formed oil with very high viscosity but often contains impurities such as sulfur. While conventional light oil has viscosities ranging from about 0.5 centipoise (cP) to about 100 cP, heavy oil has a viscosity that ranges from 100 cP to over 1,000,000 cP. Heavy oil reserves are estimated to equal about fifteen percent of the total remaining oil resources in the world. In the United States alone, heavy oil resources are estimated at about 30.5 billion barrels and heavy oil production accounts for a substantial portion of domestic oil production. For example, in California alone, heavy oil production accounts for over sixty percent of the states total oil production. With reserves of conventional light oil becoming more difficult to find, improved methods of heavy oil extractions have become more important. Unfortunately, heavy oil is typically expensive to extract and recovery is much slower and less complete than for lighter oil reserves. Therefore, there is a compelling need to develop a more efficient and effective means for extracting heavy oil.
Viscous oil that is too deep to be mined from the surface may be heated with hot fluids or steam to reduce the viscosity sufficiently for recovery by production wells. A variety of processes are used to recover viscous hydrocarbons, such as heavy crude oils and bitumen, from underground deposits. There are extensive deposits of viscous hydrocarbons throughout the globe, including large deposits in the Northern Alberta tar sands that are not recoverable with traditional oil well production technologies. A problem associated with producing hydrocarbons from such deposits is that the hydrocarbons are too viscous to flow at commercially viable rates at the temperatures and pressures present in the reservoir. In some cases, these deposits are mined using open-pit mining techniques to extract the hydrocarbon-bearing material for later processing to extract the hydrocarbons.
Alternatively, thermal techniques may be used to heat the reservoir fluids and rock to produce the heated, mobilized hydrocarbons from wells. One such technique for utilizing a single well for injecting heated fluids and producing hydrocarbons is described in U.S. Pat. No. 4,116,275, which also describes some of the problems associated with the production of mobilized viscous hydrocarbons from horizontal wells.
One thermal method of recovering viscous hydrocarbons using two vertically spaced wells is known as steam-assisted gravity drainage (SAGD) process. The SAGD process is currently the only commercial process that allows for the extraction of bitumen at depths too deep to be strip-mined. Various embodiments of the SAGD process are described in Canadian Patent No. 1,304,287 and corresponding U.S. Pat. No. 4,344,485. In the SAGD process, steam is pumped through an upper, horizontal injection well into a viscous hydrocarbon reservoir while the heated, mobilized hydrocarbons are produced from a lower, parallel, horizontal production well vertically spaced proximate to the injection well. The injection and production wells are typically located close to the bottom of the hydrocarbon deposits.
The SAGD process is believed to work as follows. The injected steam creates a “steam chamber” in the reservoir around and above the horizontal injection well. As the steam chamber expands upwardly and laterally from the injection well, viscous hydrocarbons in the reservoir are heated and mobilized, especially at the margins of the steam chamber where the steam condenses and heats a layer of viscous hydrocarbons by thermal conduction. The heated, mobilized hydrocarbons (and steam condensate) drain under the effects of gravity towards the bottom of the steam chamber, where the production well is located. The mobilized hydrocarbons are collected and produced from the production well. The rate of steam injection and the rate of hydrocarbon production may be modulated to control the growth of the steam chamber to ensure that the production well remains located at the bottom of the steam chamber and in a position to collect the mobilized hydrocarbons.
In order to initiate a SAGD production, thermal communication must be established between an injection and a production SAGD well pair. Initially, the steam injected into the injection well of the SAGD well pair will not have any effect on the production well until at least some thermal communication is established because the hydrocarbon deposits are so viscous and have little mobility. Accordingly, a start-up phase is required for the SAGD operation. Typically, the start-up phase takes about three months before thermal communication is established between the SAGD well pair, depending on the formation lithology and the actual inter-well spacing.
The traditional approach to starting-up the SAGD process is to simultaneously operate the injection and production wells independently of one another to circulate steam. The injection and production wells are each completed with a screened (porous) casing (or liner) and an internal tubing string extending to the end of the liner, forming an annulus between the tubing string and casing. High pressure steam is simultaneously injected through the tubing string of both the injection and production wells. Fluid is simultaneously produced from each of the injection and production wells through the annulus between the tubing string and the casing. In effect, heated fluid is independently circulated in each of the injection and production wells during the start-up phase, heating the hydrocarbon formation around each well by thermal conduction. Independent circulation of the wells is continued until efficient thermal communication between the wells is established. In this way, an increase in the fluid transmissibility through the inter-well span between the injection and production wells is established by conductive heating. The pre-heating stage typically takes about three to four months. Once sufficient thermal communication is established between the injection wells, the upper, injection well is dedicated to steam injection and the lower production well is dedicated to fluid production.
There are several patents on the improvements to SAGD operation. U.S. Pat. No. 6,814,141 describes applying vibrational energy in a well fracture to improve SAGD operation. U.S. Pat. No. 5,899,274 teaches addition of solvents to improve oil recovery. U.S. Pat. No. 6,544,411 describes decreasing the viscosity of crude oil using ultrasonic source. U.S. Pat. No. 7,091,460 claims in situ, dielectric heating using variable radio frequency waves.
In a recent patent publication (U.S. Patent Publication 20070289736/US-A1, filed May 25, 2007), it is disclosed to extract hydrocarbons from a target formation, such as a petroleum reservoir, heavy oil, and tar sands by utilizing microwave energy to fracture the containment rock and for liquification or vitalization of the hydrocarbons.
In another recent patent publication (US Patent Publication 20070131591/US-A1, filed Dec. 14, 2006), it is disclosed that lighter hydrocarbons can be produced from heavier carbon-base materials by subjecting the heavier materials to microwave radiations in the range of about 4 GHz to about 18 GHz. This publication also discloses extracting hydrocarbons from a reservoir where a probe capable of generating microwaves is inserted into the oil wells and the microwaves are used to crack the hydrocarbons with the cracked hydrocarbon thus produced being recovered at the surface.
Despite these disclosures, it is unlikely that direct microwave cracking or heating of hydrocarbons would be practical or efficient. It is known that microwave energy is absorbed by a polar molecule with a dipole moment and bypasses the molecules that lack dipole moment. The absorption of the microwave energy by the polar molecule causes excitation of the polar molecule thereby transforming the microwave energy into heat energy (known as the coupling effect). Accordingly, when a molecule with a dipole moment is exposed to microwave energy it gets selectively heated in the presence of non-polar molecules. Generally, heavy oils comprise non-polar hydrocarbon molecules; accordingly, hydrocarbons would not get excited in the presence of microwaves.
Additionally, while the patent publication above claims to break the hydrocarbon molecules, the energy of microwave photons is very low relative to the energy required to cleave a hydrocarbon molecule. Thus, when hydrocarbons are exposed to microwave energy, it will not affect the structure of a hydrocarbon molecule. (See, for example, “Microwave Synthesis”, CEM Publication, 2002 by Brittany Hayes).